Growth and Differentiation Factor-8 (GDF-8), also known as myostatin, and TGF-β1 are members of the Transforming Growth Factor-beta (TGF-β) superfamily of structurally related growth factors, all of which possess physiologically important growth-regulatory and morphogenetic properties (Kingsley et al. (1994) Genes Dev., 8: 133-46; Hoodless et al. (1998) Curr. Topics Microbiol. Immunol., 228: 235-72). For example, activation of TGF-β1 signaling and expansion of extracellular matrix are early and persistent contributors to the development and progression of fibrotic disorders, such as involved in chronic renal disease and vascular disease. Border W. A., et al, N. Engl. J. Med., 1994; 331(19), 1286-92. GDF-8 is a negative regulator of skeletal muscle mass. For example, GDF-8 is highly expressed in the developing and adult skeletal muscle. The GDF-8 null mutation in transgenic mice is characterized by a marked hypertrophy and hyperplasia of the skeletal muscle (McPherron et al. (1997) Nature, 387: 83-90). Similar increases in skeletal muscle mass are evident in naturally occurring mutations of GDF-8 in cattle (Ashmore et al. (1974) Growth, 38: 501 507; Swatland and Kieffer (1994) J. Anim. Sci., 38: 752-757; McPherron and Lee (1997) Proc. Natl. Acad. Sci. USA, 94: 12457-12461; and Kambadur et al. (1997) Genome Res., 7: 910-915). Because GDF-8 is expressed in both developing and adult muscles, it is not clear whether it regulates muscle mass during development or in adults. Recent studies have also shown that muscle wasting associated with HIV-infection in humans is accompanied by increases in GDF-8 protein expression (Gonzalez-Cadavid et al. (1998) PNAS, 95: 14938-43). In addition, GDF-8 can modulate the production of muscle-specific enzymes (e.g., creatine kinase) and modulate myoblast cell proliferation (WO 00/43781).
A number of human and animal disorders are associated with loss or functional impairment of muscle tissue, including muscular dystrophy, muscle atrophy, congestive obstructive pulmonary disease, muscle wasting syndrome, sarcopenia, and cachexia. To date, very few reliable or effective therapies exist for these disorders. However, the terrible symptoms associated with these disorders may be substantially reduced by employing therapies that increase the amount of muscle tissue in patients suffering from the disorders. While not curing the conditions, such therapies would significantly improve the quality of life for these patients and could ameliorate some of the effects of these diseases.
In addition to its growth-regulatory and morphogenetic properties in skeletal muscle, GDF-8 may also be involved in a number of other physiological processes, including glucose homeostasis in the development of type 2 diabetes and adipose tissue disorders, such as obesity. For example, GDF-8 modulates pre-adipocyte differentiation to adipocytes (Kim et al. (2001) BBRC, 281: 902-906).
There are also a number of conditions associated with a loss of bone, including osteoporosis, especially in the elderly and/or postmenopausal women. Currently available therapies for these conditions work by inhibiting bone resorption.
Like TGF-β-1, -2, and -3, the GDF-8 protein is synthesized as a precursor protein consisting of an amino-terminal propeptide and a carboxy-terminal mature domain (McPherron and Lee, (1997) Proc. Natl. Acad. Sci. USA, 94: 12457-12461). Before cleavage, the precursor GDF-8 protein forms a homodimer. The amino-terminal propeptide is then cleaved from the mature domain. The cleaved propeptide may remain noncovalently bound to the mature domain dimer, inactivating its biological activity (Miyazono et al. (1988) J. Biol. Chem., 263: 6407-6415; Wakefield et al. (1988) J. Biol. Chem., 263; 7646-7654; and Brown et al. (1990) Growth Factors, 3: 35-43). It is believed that two GDF-8 propeptides bind to the GDF-8 mature dimer (Thies et al. (2001) Growth Factors, 18: 251-259). Due to this inactivating property, the propeptide is known as the “latency-associated peptide” (LAP), and the complex of mature domain and propeptide is commonly referred to as the “small latent complex” (Gentry and Nash (1990) Biochemistry, 29: 6851-6857; Derynck et al. (1995) Nature, 316: 701-705; and Massague (1990) Ann. Rev. Cell Biol., 12: 597-641). Other proteins are also known to bind to GDF-8 or structurally related proteins and inhibit their biological activity. Such inhibitory proteins include follistatin, and potentially, follistatin-related proteins (Gamer et al. (1999) Dev. Biol., 208: 222-232). The mature domain is believed to be active as a homodimer when the propeptide is removed.
GDF-8 is highly conserved in sequence and in function across species. The amino acid sequence of murine and human GDF-8 is identical, as is the pattern of mRNA expression (McPherron et al. (1997) Nature 387: 83-90; Gonzalez-Cadavid et al. (1998) Proc. Natl. Acad. Sci. USA 95: 14938-14943). This conservation of sequence and function suggests that inhibition of GDF-8 in humans is likely to have a similar effect to inhibition of GDF-8 in mice.
U.S. Pat. No. 7,320,789 shows that GDF-8 antibodies in mouse models can increase muscle strength (e.g., for treating sarcopenia), increase muscle mass and strength in dystrophic muscle (e.g., for treating Duchenne's muscular dystrophy), increase bone mass and bone density (e.g., for prevention and treatment of osteoporosis), augment bone healing (e.g., for treating an established muscle or bone degenerative disease (e.g., fracture repair and spine fusion, preventing the decline in bone mass, microarchitecture and strength associated with estrogen deficiency, increasing trabecular bone density), and are useful for treatment of metabolic disorders such as type 2 diabetes, impaired glucose tolerance, metabolic syndrome (e.g., syndrome X), insulin resistance induced by trauma (e.g., burns), and adipose tissue disorders (e.g., obesity).